The present invention relates to a power control circuit for providing a substantially constant intensity light source and a corresponding method using this control circuit.
By way of background, traffic signal lamps typically use either incandescent or LED (light-emitting diode) lamps. LED traffic signals are more reliable, more mechanically stable, safer, more energy efficient and more environmentally friendly than incandescent lamps. Thus, LED traffic signals are gaining in popularity.
The voltage and current characteristics of an LED lamp are sensitive to temperature. The LEDs used will have a forward voltage specified at an intended operating current. In particular, the forward voltage changes with the temperature, and, consequently, the current follows the variation. Thus, if the forward voltage increases, then the forward current will decrease. Likewise, if the forward voltage decreases, then the forward current increases.
For example, for a given type of LED widely used in the fabrication of traffic lights and signals, rail signals, signage, commercial refrigeration lighting, general Illumination, vehicle lighting, variable message and many other applications, a constant voltage of 1.8 volts will produce in the LED a current of about 7.5 mA at a temperature of −25° C., a current of about 20.5 mA at a temperature of +25° C., and a current of about 30 mA at a temperature of +60° C. The magnitude of the current through the light-emitting diode at a temperature of +60° C. is therefore, for a constant voltage of 1.8 volt, about 1.6 times higher than the magnitude of the current at a temperature of +25° C.
A constant voltage may be maintained such that the voltage across the LEDs is constant for all environments (e.g., −40 to 74° C.). It is known that at high temperatures the forward voltage of the LEDs decreases, and because the driver or the power supply maintains the voltage across the LEDs constant, the LED current will increase exponentially and stress the LEDs (bright LEDs).
At low temperatures the forward voltage of the LEDs increases, and because the driver of the power supply maintains the voltage across the LEDs constant, the LED current will decrease exponentially and the light will be dim (dim LEDs). Therefore, voltage feedback control may be detrimental to the service life of such an LED.
Also, a fixed LED output current presents the following drawbacks: at higher temperature the LED forward voltage decreases and then the output LED power decreases, which means light out decreases; and at lower temperatures the LED forward voltage increases and then the output LED power increases, which means light out increases.
Thus, there is a need for a device and method that eliminates the above-discussed drawbacks of the prior art by regulating the output power, and hence the light intensity, of non-linear light emitting loads such as light-emitting diodes.